Integrins are a class of cellular receptors known to bind extracellular matrix proteins, and therefore mediate cell-cell and cell-extracellular matrix interactions, referred generally to cell adhesion events. However, although many integrins and the ligands that bind an integrin are described in the literature, the biological function of many of the integrins remains elusive. The integrin receptors constitute a family of proteins with shared structural characteristics of noncovalent heterodimeric glycoprotein complexes formed of α and β subunits.
The vitronectin receptor, named for its original characteristic of preferential binding to vitronectin, is now known to refer to three different integrins, designated αvβ1 αvβ3 and αvβ5. Horton, Int. J. Exp. Pathol., 71:741-759 (1990). αvβ1 binds fibronectin and vitronectin. αvβ3 binds a large variety of ligands, including fibrin, fibrinogen, laminin, thrombospondin, vitronectin, von Willebrand's factor, osteospontin and bone sialoprotein I. αvβ5 binds vitronectin. The specific cell adhesion roles these three integrins play in the many cellular interaction in tissues is still under investigation, but it is clear that there are different integrins with different biological functions.
One important recognition site in the ligand for many integrins is the arginine-glycine-aspartic acid (RGD) tripeptide sequence. RGD is found in all of the ligands identified above for the vitronectin receptor integrins. This RGD recognition site can be mimicked by polypeptides (“peptides”) that contain the RGD sequence, and such RGD peptides are known inhibitors of integrin function. It is important to note, however, that depending upon the sequence and structure of the RGD peptide, the specificity of the inhibition can be altered to target specific integrins.
For discussions of the RGD recognition site, see Pierschbacher et al., Nature, 309:30-33 (1984), and Pierschbacher et al., Proc. Natl. Acad. Sci. USA, 81:5985-5988 (1984). Various RGD polypeptides of varying integrin specificity have also been described by Grant et al., Cell, 58:933-943 (1989), Ruggeri et al., and Aumailley et al., FEBS Letts., 291:50-54 (1991), and in U.S. Pat. Nos. 4,517,686, 4,578,079, 4,589,881, 4,614,517, 4,661,111, 4,792,525, 4,683,291, 4,879,237, 4,988,621, 5,041,380 and 5,061,693.
Angiogenesis is a process of tissue vascularization that involves the growth of new developing blood vessels into a tissue, and is also referred to as neo-vascularization. The process is mediated by the infiltration of endothelial cells and smooth muscle cells. The process is believed to proceed in any one of three ways: The vessels can sprout from pre-existing vessels, de-novo development of vessels can arise from precursor cells (vasculogenesis), or existing small vessels can enlarge in diameter. Blood et al., Bioch. Biophys. Acta, 1032:89-118 (1990). Vascular endothelial cells are known to contain at least five RGD-dependent integrins, including the vitronectin receptor (αvβ3 or αvβ5), the collagen Types I and IV receptor (α1β1) the laminin receptor (α2β1), the fibronectin/laminin/collagen receptor (α3β1) and the fibronectin receptor (α5β1). Davis et al., J. Cell. Biochem., 51:206-218 (1993). The smooth muscle cell is known to contain at least six RGD-dependent integrins, including α5β1, αvβ3 and αvβ5.
Angiogenesis is an important process in neonatal growth, but is also important in wound healing and in the pathogenesis of a large variety of clinical diseases including tissue inflammation, arthritis, tumor growth, diabetic retinopathy, macular degeneration by neovascularization of retina and the like conditions. These clinical manifestations associated with angiogenesis are referred to an angiogenic diseases. Folkman et al., Science, 235:442-447 (1987). Angiogenesis is generally absent in adult or mature tissues, although it does occur in wound healing and in the corpeus leuteum growth cycle. See, for example, Moses et al., Science, 248:1408-1410 (1990).
It has been proposed that inhibition of angiogenesis would be a useful therapy for restricting tumor growth. Inhibition of angiogenesis has been proposed by (1) inhibition of release of “angiogenic molecules” such as βFGF, (2) neutralization of angiogenic molecules, such as by use of anti-βFGF antibodies, and (3) inhibition of endothelial cell response to angiogenic stimuli. This latter strategy has received attention, and Folkman et al., Cancer Biology, 3:89-96 (1992), have described several endothelial cell response inhibitors, including collagenase inhibitor, basement membrane turnover inhibitors, angiostatic steroids, fungal-derived angiogenesis inhibitors, platelet factor 4, thrombospondin, arthritis drugs such as D-penicillamine and gold thiomalate, vitamin D3 analogs, alpha-interpheron, and the like that might be used to inhibit angiogenesis. For additional proposed inhibitors of angiogenesis, see Blood et al., Bioch. Biophys. Acta., 1032:89-118 (1990), Moses et al., Science, 248:1408-1410 (1990), Ingber et al., Lab. Invest., 59:44-51 (1988), and U.S. Pat. Nos. 5,092,885, 5,112,946, 5,192,744, and 5,202,352. None of the inhibitors of angiogenesis described in the foregoing references are targeted at inhibition of αvβ3.
RGD-containing peptides that inhibit vitronectin receptor αvβ3 have also been described. Aumailley et al., FEBS Letts., 291:50-54 (1991), Choi et al., J. Vasc. Surg., 19:125-134 (1994), and Smith et al., J. Biol. Chem., 265:12267-12271 (1990). However, the role of the integrin αvβ3 in angiogenesis has never been suggested nor identified until the present invention.
Inhibition of cell adhesion in vitro using monoclonal antibodies immunospecific for various integrin α or β subunits have implicated αvβ3 in cell adhesion of a variety of cell types including microvascular endothelial cells. Davis et al., J. Cell. Biol., 51:206-218 (1993). In addition, Nicosia et al., Am. J. Pathol., 138:829-833 (1991), described the use of the RGD peptide GRGDS (SEQ ID NO 15) to in vitro inhibit the formation of “microvessels” from rat aorta cultured in collagen gel. However, the inhibition of formation of “microvessels” in vitro in collagen gel cultures is not a model for inhibition of angiogenesis in a tissue because it is not shown that the microvessel structures are the same as capillary sprouts or that the formation of the microvessel in collagen gel culture is the same as neo-vascular growth into an intact tissue, such as arthritic tissue, tumor tissue or disease tissue where inhibition of angiogenesis is desirable.
Therefore, other than the studies reported here, Applicants are unaware of any other demonstration that angiogenesis could be inhibited in a tissue using inhibitors of cell adhesion. In particular, it has never been previously demonstrated that αvβ3 function is required for angiogenesis in a tissue or that αvβ3 antagonists can inhibit angiogenesis in a tissue.